Avoid enumerating certificates with non-matching key type

If the key type was specified but the ID was NULL or matched a subject, it
was possible that a certificate was returned that didn't actually match
the requested key type.

Closes strongswan/strongswan#141.

src/libcharon/plugins/stroke/stroke_ca.c

@@ -208,6 +208,11 @@ CALLBACK(certs_filter, bool,
 					return TRUE;
 				}
 			}
+			else
+			{
+				public->destroy(public);
+				continue;
+			}
 			public->destroy(public);
 		}
 		else if (data->key != KEY_ANY)

src/libstrongswan/credentials/sets/mem_cred.c

@@ -108,6 +108,11 @@ CALLBACK(certs_filter, bool,
 					return TRUE;
 				}
 			}
+			else
+			{
+				public->destroy(public);
+				continue;
+			}
 			public->destroy(public);
 		}
 		else if (data->key != KEY_ANY)